A voltage converter converts an input voltage into a desired output voltage with control to the duty ratio of its switches, and the switching frequencies of the voltage converter are typically determined by a clock circuit. In some applications, clock signals generated by the clock circuit may either be regulated by discrete resistors or be synchronized with an external clock signal. Here lies a problem, however, that a voltage drop at the output terminal of the voltage converter will occur at the moment the clock signal is synced with the external clock signal. Consequently, an improved clock circuit is needed which can steady the output voltage of the voltage converter when clock signals are shifted.